The present invention relates to a method of driving passive matrix displays and, more particularly, to a method of driving displays based upon phosphorescent organic light emitting diode materials.
FIG. 1 is a block diagram of a conventional display addressing system 100. The addressing system 100 may be populated by a display matrix 110, a row driver 120 and a column driver 130. The display matrix may 110 include an array of picture elements (xe2x80x9cpixelsxe2x80x9d) (not shown) typically organized into a regular array of columns and rows. Each pixel row may be accessed electrically by a row line (collectively labeled 125) and each pixel column may be accessed electrically by a column line (collectively labeled 135). To activate a pixel, the pixel""s row line of the pixel typically carries an electrical excitation signal; its column line typically carries data corresponding to the desired display output.
FIG. 2 is a timing diagram illustrating a conventional method of driving the display matrix 110 of FIG. 1. As is known, display information typically is organized into frames. The display data of a first frame is rendered on the display matrix, cleared, and the display data of a second frame is rendered thereafter. With conventional frame rates may span from 10-30 frames per second, a frame period may span from 30 to 100 ms.
Conventionally in modern displays, each row of a display matrix is driven with an excitation pulse having a duration of 1/Nth of a frame, where N is the number of rows in the display matrix 110. During this excitation pulse, the column driver generates data signals corresponding to the information content that should be displayed on the respective row. When the excitation pulse concludes, the row driver 120 advances to a subsequent row and applies the excitation pulse. The process repeats for every row in the display matrix. Each row receives only a single excitation pulse per frame.
Light emitting devices, when activated, typically emit light during the excitation pulse. The light output of these devices typically decays much faster than the frame period of the display. Human beings tend to perceive the output of the display as a time average of the light output over the entire frame. Thus, to achieve sufficient brightness, the light emitting devices typically are driven with very high voltages that cause the devices to emit a very strong light output to achieve a predetermined perceived brightness. Typically materials that are chosen for such displays exhibit a linearity between the excitation potential used and the light output that the material generatesxe2x80x94if one were to double the excitation potential, the material typically generates twice the light output. This is a well-known characteristic of displays.
Recent advances in material science have developed a new class of light emitting devices based on organic materials. These xe2x80x9corganic light emitting devicesxe2x80x9d (or, xe2x80x9cOLEDsxe2x80x9d) may include light emitting devices whose luminescence is based on emission from long-lived phosphor dopants. OLEDs using phosphors are beneficial because they tend to be highly efficient compared to those employing more conventional fluorescent dopants. In contrast to fluorescent dopants, phosphors tend energize very quickly but decay rather slowly. OLEDs, however, are current driven rather than voltage driven devices. Phosphor-doped OLEDs do not exhibit the linearity described above with respect to other materials. The materials reach a point that they will not generate any increased light output no matter how hard the material is driven. Indeed, over-driving of the OLED displays (even in the case of fluorescent doped OLEDs) simply may damage the light emitting devices themselves and reduce the useful life of the display. The maximum light output of some of these organic materials is insufficient to generate sufficiently bright output to be useful in a display.
Notwithstanding the problems associated with the phosphor doped OLEDs, they possess remarkable other advantages for use in displays. For example, OLEDs may be stacked, a property that suggests that OLEDs can be applied in very compact display designs. Accordingly, there remains a significant commercial interest in the development of OLEDs for use in display devices.
There is a need in the art for a display driving method for OLED displays that generates higher light output further, there is a need in the art for a display driving method that drives organic light emitting devices at lower current levels and yet achieves increased brightness.
Embodiments of the present invention provide an addressing method that induces increased light output in an organic light emitting display by applying several excitation currents to each row in an display per frame. The row excitation pulses may advance sequentially across every row in the display and, when the row driver reaches the last row in the display, the row driver returns to the first row in the display and begins again. In an embodiment, the row driver may complete 10-100,000 cycles across all rows in the display for each frame. This method of addressing the display yields increase light output with a correspondingly lower-powered excitation current.